The present invention relates to an acid component-removing agent for a gas, a method for producing it and method for removing acid components from a gas.
It has been known to use slaked lime as an acid component-removing agent, to absorb and remove hydrogen chloride or a sulfur oxide in an exhaust gas discharged from e.g. a refuse incinerator. In such a case, slaked lime is dispersed in an exhaust gas discharge path from the incinerator at a region having temperatures of from 150 to 300xc2x0 C., followed by collection by a bag filter or an electrical dust collector, whereby acid components are removed.
However, it is necessary to use slaked lime in an excessive amount of from 3 to 4 equivalent amount to reaction equivalent, whereby the amount of dust to be wasted will increase. Further, the dust is solidified by means of concrete, and the solidified dust is accounting for the dumping site for final disposal. The reaction product of hydrogen chloride and slaked lime is a calcium chloride which is water-soluble, whereby the reaction product can be removed by dissolving in water. However, some of slaked lime excessively added will form quick lime which is water-insoluble, and accordingly only a small amount of effects of reducing dust by water will be obtainable. Further, calcium scales will form during treatment of an aqueous calcium chloride solution at the final disposal site, thus causing troubles.
Further, it has been known to use sodium hydrogencarbonate as the acid component-removing agent instead of slaked lime. In such a case, unreacted sodium hydrogencarbonate will form sodium carbonate which is water-soluble, and accordingly it is effective to reduce dust. For example, EP0740577 discloses an acid component-removing agent comprising a composition containing sodium hydrogencarbonate in an amount exceeding 98 wt % and sodium carbonate in an amount of less than 2 wt %. It is disclosed that the mean particle diameter of the composition is at most 50 xcexcm, preferably from 10 to 30 xcexcm.
However, sodium hydrogencarbonate is expensive as compared with slaked lime, and it has thereby been desired to supply sodium hydrogencarbonate which has a high reactivity and presents effects with a small amount, with an industrial scale at a low cost. Further, it is desirable that the acid component-removing agent can be used without installation of a dust arrestor, it can be easily and stably injected to a gas to be treated, it can be well dispersed in the gas, the reaction rate is high, and a long-term preservation in a storage tank or a stock room at the operation site is possible.
Further, sulfur trioxide contained in an exhaust gas produced e.g. during operation of a boiler with fuel containing a sulfur content, will react with water vapor contained in the exhaust gas to form a sulfuric acid mist, which will form white smoke, violet smoke or white fume when discharged in the air, and will cause air pollution. Accordingly, in order to remove the sulfur trioxide component, it has been employed to preliminarily add a slurry having e.g. an oxide or hydroxide of calcium or magnesium dispersed in an organic solvent to the fuel, for prevention of sulfur trioxide formation, or to neutralize sulfur trioxide after combustion. However, with such methods, the additions are likely to deposit on a heat exchanger in the boiler, and when a large amount of the additives deposit thereon, the operation of the boiler will be hindered, and accordingly it is hard to use a large amount of the additives.
Further, to actively remove sulfur trioxide in a flue, it has been employed to inject a powder of the above-mentioned oxide or hydroxide or a slurry thereof, to the flue after the exhaust gas has passed through an air preheater. However, in the case where the powder itself is injected by this method, as a fine powder having a poor fluidity is discharged from the storage tank, and the powder is injected by means of e.g. a screw feeder as a positive-displacement powder transportation apparatus, quantitativeness will be poor, and no stable effect is likely to be obtainable. Further, in the case of injecting the slurry, the powder contained in the slurry is likely to deposit on and clog the transportation line for injecting the slurry, whereby the method can hardly be carried out stably.
As the acid component-removing agent for e.g. a refuse incineration plant, slaked lime has conventionally been used. Although slaked lime can be available at a low cost, there are problems in view of waste disposal, such as increase in water-insoluble dust to be wasted, or generation of calcium scales in the seepage water at the final disposal site. Further, in the case of using sodium hydrogencarbonate, as it is expensive, it is necessary to increase the reaction efficiency and to reduce the amount to be used. Further, when the amount to be used can be reduced, an equipped apparatus can be downsized, and the dust to be treated can be reduced.
On the other hand, in the case of using e.g. magnesium oxide to remove acid components in a boiler exhaust gas, such as sulfur trioxide or a sulfuric acid mist based on sulfur trioxide (hereinafter they will generically be referred to as SO3 component), since magnesium oxide also has a low reaction efficiency, it is necessary to add an excessive amount of magnesium oxide. In such a case, unreacted magnesium oxide will remain in the flue, and may cause problems in post-treatment, since magnesium oxide has a low solubility in water. Further, it is difficult to quantify the injection amount of the powder, and accordingly it has been desired to develop an acid component-removing agent which can be sprayed on the flue stably and quantitatively.
Under these circumstances, it is an object of the present invention to industrially provide an acid component-removing agent which can effectively remove acid components such as hydrogen chloride, sulfur oxide such as SO3, nitrogen oxide or hydrogen fluoride from an exhaust gas, with which waste disposal can easily be carried out, and which can reduce the amount of waste.
The present invention provides an acid component-removing agent which comprises sodium hydrogencarbonate having a volume-based mean particle diameter of from 1 to 9 xcexcm as measured by a laser diffraction and scattering method, and a method for producing it.
Of the acid component-removing agent of the present invention, the mean particle diameter of the sodium hydrogencarbonate is represented by the value of the volume-based mean particle diameter as measured by using a laser diffraction and scattering particle size analyzer. Hereinafter the mean particle diameter will be represented by the value as measured by this method, unless otherwise specified. In the present specification, the mean particle diameter is measured by using Microtrack FRA9220 manufactured by NIKKISO CO., LTD. For a supplementary examination, a volume-based mean particle diameter means the same as a mass-based mean particle diameter, because sodium hydrogencarbonate has a homogeneous density.
The acid component-removing agent of the present invention reacts with acid components present in a gas, and removes the acid components from said gas. The acid components to be removed are not particularly limited, and the acid component-removing agent of the present invention can be applied to various acid components such as hydrogen chloride, sulfur dioxide, sulfur trioxide, hydrogen fluoride and nitrogen oxides. Particularly when the acid component-removing agent of the present invention is applied to a compound containing chlorine such as hydrogen chloride, or to a SO3 component, the reaction efficiency is high, and it is easy to handle it, as compared with a conventional one. Now, the present invention will be explained with reference to hydrogen chloride. However, the same applies to the other acid components.
By using sodium hydrogencarbonate, when sodium hydrogencarbonate particles are converted to sodium carbonate by calcination, the particles become porous, and effectively react with hydrogen chloride, whereby hydrogen chloride can effectively be removed. The present inventors have measured and observed the particle sizes, pore distributions and shapes of sodium hydrogencarbonate and sodium carbonate before and after calcination in detail, and they have found that the reaction characteristics of the porous structure obtained when the particles become porous, with hydrogen chloride, are sensitively affected by the average particle size of the sodium hydrogencarbonate, and the present invention has been accomplished.
In the acid component-removing agent of the present invention, the mean particle diameter of the sodium hydrogencarbonate is within a range of from 1 to 9 xcexcm. If the mean particle diameter of the sodium hydrogencarbonate exceeds 9 xcexcm, when the sodium hydrogencarbonate is calcinated to form sodium carbonate, spongy sodium carbonate particles having fine pores with diameters of at most several tens nm, while maintaining the profile of the outer surface of the sodium hydrogencarbonate, will be obtained. On the other hand, if the mean particle diameter of the sodium hydrogencarbonate is at most 9 xcexcm, pores having relatively large diameters to the particle sizes will be formed, and accordingly the surfaces of the particles are significantly rugged, and irregular profiles will be obtained.
The feature of the present invention resides in such a fact that sodium hydrogencarbonate having a mean particle diameter of at most 9 xcexcm is calcinated to produce sodium carbonate, the above-mentioned special pore structure can be obtained, and said pore structure is extremely effective for absorption of an acid gas. In the prior art, although an example for absorption of an acid gas by sodium hydrogencarbonate having a large particle size has been disclosed, sodium hydrogencarbonate having a mean particle diameter of at most 9 xcexcm has not been employed, and it has not been known that the pore structure and the particle shape of the sodium carbonate after calcination change when the particle size of the sodium hydrogencarbonate is made small.
The present inventors have estimated that when the particles are scattered in the air current, or when the particles are trapped by a bag filter to form a particle deposit bed, and gases of the acid components such as hydrogen chloride pass through the particles, diffusion of the acid components into the surfaces of particles and the reaction between the acid components with the particles are likely to take place by the above-mentioned shape, whereby a good reaction with the acid components can be accelerated. The mean particle diameter of the sodium hydrogencarbonate is more preferably at most 8 xcexcm.
The lower limit of the mean particle diameter of the sodium hydrogencarbonate is not particularly limited in view of reactivity with the acid components. However, if the mean particle diameter is less than 1 xcexcm, the particles tend to adhere to one another, and no adequate fluidity may be maintained even if an anti-caking agent as mentioned hereinafter is used together, or it may be necessary to add a large amount of the anti-caking agent. Further, for industrial production, costs for operation and equipments required for grinding may be excessive.
The sodium hydrogencarbonate in the acid component-removing agent of the present invention, has a pore distribution in its powder bed as measured by a mercury penetration method, wherein the volume of pores having pore diameters within a range of from 1 to 10 xcexcm is preferably at least 0.4 cm3/g. In the present specification, the pore distribution in the powder bed as measured by a mercury penetration method, is represented by the value as measured by a mercury penetration method with respect to the powder bed of the sodium hydrogencarbonate. Specifically, measurement is carried out with respect to a powder bed filled by lightly shaking off 0.25 g of the powder from a spatula in a cylindrical cell with a diameter of 15 mm and a height of 30 mm. Hereinafter the volume of pores will be represented by the value as measured by this method, unless otherwise specified.
Further, the sodium hydrogencarbonate in the acid component-removing agent of the present invention is preferably such that when it is calcinated at 200xc2x0 C. for 1 hour, the resulting sodium carbonate has the same powder characteristics as the sodium hydrogencarbonate. Namely, the obtained sodium carbonate has a pore distribution in its powder bed as measured by a mercury penetration method, wherein the volume of pores having pore diameters within a range of from 1 to 10 xcexcm is preferably at least 0.4 cm3/g.
The calcination operation to convert the sodium hydrogencarbonate into sodium carbonate is carried out in such a manner that 5 g of the sodium hydrogencarbonate is thinly scattered on a laboratory dish with a diameter of 60 mm, preliminarily heated to about 200xc2x0 C., which is then left in a hot-air circulation drier maintained at 200xc2x0 C., and the laboratory dish was taken out after 1 hour.
When the volume of pores of the sodium hydrogencarbonate and the sodium carbonate having pore diameters within a range of from 1 to 10 xcexcm, is within the above-specified range, a high effect for removing the acid components can be obtained. Although the mechanism of appearance of the effect has not been clearly understood, it is considered that easiness in diffusion of the acid components in the acid component-removing agent is correlated with the volume of pores having pore diameters within a range of from 1 to 10 xcexcm. Namely, it is considered that in the acid component-removing agent of the present invention, as the mean particle diameter of the sodium hydrogencarbonate is at most 9 xcexcm, when the sodium hydrogencarbonate is converted to sodium carbonate, pores having large diameters of at least 1 xcexcm are likely to be formed, whereby the particles will significantly be rugged, and will present irregular profiles, and accordingly diffusion of the gas will easily take place.
If the mean particle diameter of the sodium hydrogencarbonate exceeds 9 xcexcm, in the resulting sodium carbonate, pores having diameters of at least 1 xcexcm will not form, or will form but in a small ratio. For example, when sodium hydrogencarbonate having a mean particle diameter of 83 xcexcm is calcinated at 200xc2x0 C. for 1 hour, the resulting sodium carbonate has a volume of pores having pore diameters of from 0.1 to 1.0 xcexcm of 0.30 cm3/g, and a volume of pores having pore diameters of from 1.0 to 10 xcexcm of 0.04 cm3/g. Further, when sodium hydrogencarbonate having a mean particle diameter of 21 xcexcm is calcinated at 200xc2x0 C. for 1 hour, the resulting sodium carbonate has a volume of pores having pore diameters of from 0.1 to 1.0 xcexcm of 0.28 cm3/g, and a volume of pores having pore diameters of from 1.0 to 10 xcexcm of 0.24 cm3/g.
Namely, most of pores have pore diameters of from 0.1 to 1.0 xcexcm, and the shape is such that the particles have relatively fine pores. Accordingly, the acid components are required to diffuse in a fine and long path, whereby diffusion of the acid components will take long, such being disadvantageous to the reaction. On the other hand, volume of pores having pore diameters of from 1.0 to 10 xcexcm, which are considered to be effective to remove the acid components in a short period of time, is less than 0.4 cm3/g, and accordingly performances for absorption of the acid components tends to be poor.
When sodium hydrogencarbonate is calcinated at a temperature of at least 100xc2x0 C., it is converted into sodium carbonate. For example, with respect to sodium hydrogencarbonate and the resulting sodium carbonate obtained by calcinating the sodium hydrogencarbonate at 200xc2x0 C. for 1 hour, no significant change in the average particle size is observed before and after the calcination. Specifically, with respect to the sodium hydrogencarbonate having a mean particle diameter within a range of from 0.7 to 50 xcexcm, as observed by the present inventors, it has been confirmed that the average particle size does not significantly change before and after the calcination.
The difference in true volume between sodium hydrogencarbonate (molecular weight: 84.01, density: 2.19 g/cm3) and sodium carbonate (molecular weight: 105.99, density: 2.53 g/cm3) obtainable from said sodium hydrogencarbonate, is 0.33 cm3/g based on the weight of the sodium carbonate. Namely, when the sodium hydrogencarbonate is converted to sodium carbonate while maintaining the outer shape, the volume of pores of the sodium carbonate is 0.33 cm3/g. When the sodium carbonate reacts with hydrogen chloride to form sodium chloride (molecular weight: 58.44, density: 2.161 g/cm3), the true volume will slightly increase, whereby the volume of pores will decrease. However, a volume of pores of 0.19 cm3/g still remains according to calculation. This is one reason why sodium hydrogencarbonate has a high reactivity as compared with a calcium type acid component-removing agent such as slaked lime, and is essentially advantageous.
With respect to a conventional calcium type acid component-removing agent, the acid component-removing agent excessively used during the process for treating the acid components, will produce water-insoluble calcium salts in addition to calcium chloride, which will produce solid waste. On the other hand, with respect to the acid component-removing agent of the present invention, the product in the process for treating the acid components is, for example, mainly sodium chloride and sodium carbonate in the case of hydrogen chloride. Accordingly, when they are separated from ash of e.g. other heavy metals, they can be dissolved in water and treated, whereby the amount of solid waste can be reduced. Although a potassium type acid component-removing agent is also advantageous in this point, the potassium type acid component-removing agent has a high moisture absorption. Further, also in view of price for purchase, it is more advantageous to employ sodium hydrogencarbonate.
In the accompanying drawing, FIG. 1 is a flowchart illustrating a conventional method for treating an exhaust gas obtained by combustion in a boiler.
Now, the present invention will be described in detail with reference to the preferred embodiments.
The acid component-removing agent of the present invention can be produced, for example, by grinding sodium hydrogencarbonate having a mean particle diameter of at least 50 xcexcm to have a mean particle diameter of at most 9 xcexcm. The grinding method may be either of dry grinding or wet grinding.
In the case of dry grinding, it is preferred to use e.g. an impact type grinder (a grinder by means of e.g. a blade rotating at a high speed), a jet mill (a grinder by means of an impact air current) or a ball mill. It is more preferred to grind the sodium hydrogencarbonate by means of an impact type grinder equipped with an air classifier, to classify particles discharged from the grinder, and to return large particles to the grinder, since the sodium hydrogencarbonate having a desired particle size can be obtained with a high yield. Further, it is also preferred to use a jet mill, as it is suitable for grinding to obtain fine particles, and the sodium hydrogencarbonate having a desired particle size can be obtained with a high yield, without removing large particles by sieving.
In the case of wet grinding, it is preferred to use e.g. an agitator bead mill or a ball mill. It is particularly preferred to disperse sodium hydrogencarbonate in a liquid which dissolves substantially no sodium hydrogencarbonate and which will not be denaturalized to obtain a slurry, to subject said slurry to wet grinding by an agitator bead mill or a ball mill, and to separate the resulting sodium hydrogencarbonate, followed by drying, since sodium hydrogencarbonate having a small average particle size can be obtained. As the liquid dissolving substantially no sodium hydrogencarbonate, a liquid which will not be denaturalized by alkalinity of the sodium hydrogencarbonate, and which has a low viscosity, is preferred.
Such a liquid may, for example, be methanol, ethanol, acetone or C4F9OCH3. The liquid dissolving substantially no sodium hydrogencarbonate is preferably one having a solubility of the sodium hydrogencarbonate of at most 3% by mass, more preferably one having a solubility of at most 1% by mass.
The acid component-removing agent of the present invention may contain, in addition to the sodium hydrogencarbonate having a mean particle diameter of from 1 to 9 xcexcm. another acid component-removing component such as potassium hydrogencarbonate, slaked lime, calcium carbonate or zeolite, an adsorbent such as activated carbon, or an anti-caking agent such as silica or diatomaceous earth. The sodium hydrogencarbonate having a mean particle diameter of from 1 to 9 xcexcm is contained in an amount of preferably at least 70% by mass in the acid component-removing agent.
The acid component-removing agent of the present invention will coagulate when preserved for a long period of time, since sodium hydrogencarbonate having a small particle size is used for it as compared with a conventional one. Although the acid component-removing agent of the present invention can be directly supplied to a gas to be treated from a storage tank, there are possible fears that when the sodium hydrogencarbonate coagulates, fluidity as a powder tends to decrease, and discharge from the storage tank will deteriorate, or dispersion in a flue will deteriorate, whereby reactivity with the acid components will decrease. Accordingly, it is preferred to add an anti-caking agent to the acid component-removing agent. By the addition of the anti-caking agent, fluidity will be maintained, and storage of the acid component-removing agent in the storage tank can be made possible.
As the anti-caking agent, preferred is a silica powder such as fumed silica or white carbon, a basic magnesium carbonate, calcium carbonate or diatomaceous earth. Particularly preferred is a fine silicic anhydride called fumed silica, since effects can be obtained with a small amount. The content of the anti-caking agent is preferably from 0.1 to 5% by mass, particularly preferably from 0.3 to 2% by mass, based on the total amount of the acid component-removing agent including the anti-caking agent, although the optimum amount depends on the degree of grinding or storage conditions of the sodium hydrogencarbonate.
As the fumed silica, one having a hydrophobic treatment applied thereto, and a hydrophilic one having no hydrophobic treatment applied thereto, are available. When a hydrophobic silica having a hydrophobic treatment applied thereto, is used, although fluidity of the acid component-removing agent in the gas will improve, a small amount of the acid component-removing agent will float on water when dissolved in water after the treatment of the acid components. Accordingly, it is preferred to employ a hydrophilic fumed silica for e.g. treatment of an exhaust gas in a boiler equipped with a wet exhaust gas treatment apparatus.
Further, to prevent coagulation of the acid component-removing agent of the present invention, in addition to the above-mentioned methods, a method of adding from 10 to 30% by mass of sodium hydrogencarbonate having a mean particle diameter of at least 50 xcexcm, may be mentioned. The acid component-removing agent obtainable by this method preferably contains sodium hydrogencarbonate having two peaks within ranges of from 1 to 9 xcexcm and from 50 to 200 xcexcm in a volume-based mean particle diameter distribution as measured by a laser diffraction and scattering particle size analyzer, and having sodium hydrogencarbonate having particle sizes exceeding 44 xcexcm in an amount of from 10 to 30 wt % based on the total amount. Instead of measuring the amount of exceeding 44 xcexcm by a laser diffraction and scattering particle size analyzer, an amount of large size particle can be measured by a sieving method based mass percentage. Because of the crystal of sodium hydrogencarbonate is not porous or hollow, the mass percentage is able to regard the same value with the volume percentage. In concrete an amount of exceeding 45 xcexcm is measured by a sieving method employing a sieve which mesh size is 45 xcexcm. When this sieving method is employed, the acid component-removing agent of the present invention is the content of sodium hydrogencarbonate which particle size is exceeding 45 xcexcm is preferably from 10 to 30% by mass based on the acid component-removing agent, and it is preferable the size distribution frequency curve of this acid component-removing agent, measured by a laser diffraction and scattering particle size analyzer, has two peaks, one is in the range from 1 to 9 xcexcm and another is the range from 50 to 200 xcexcm.
The content of the sodium hydrogencarbonate having a mean particle diameter of from 50 to 200 xcexcm is preferably from 10 to 30% by mass based on the acid component-removing agent. The content of the sodium hydrogencarbonate having a mean particle diameter of from 50 to 200 xcexcm is less than 10% by mass, no substantial effect for improving fluidity can be obtainable. If the content of the sodium hydrogencarbonate having a mean particle diameter of from 50 to 200 xcexcm exceeds 30% by mass, efficiency for removing the acid components may deteriorate.
In the case where sodium hydrogencarbonate having such a large mean particle diameter is added, the acid component-removing agent itself (the sodium hydrogencarbonate having a mean particle diameter of from 1 to 9 xcexcm and the sodium hydrogencarbonate having a mean particle diameter of from 50 to 200 xcexcm) or a reaction product thereof with acid gasses will dissolve in water. Accordingly, a water-insoluble content after treatment of the acid components can be reduced, and generation of dust due to the anti-caking agent in a form of fine powders can be suppressed, as compared with a case of using an anti-caking agent having a low solubility in water, such as a silica powder, a basic magnesium carbonate, calcium carbonate or diatomaceous earth. Further, the sodium hydrogencarbonate having a mean particle diameter of at least 50 xcexcm can be used together with the anti-caking agent such as fumed silica. In such a case, the content of the anti-caking agent is preferably from 0.1 to 5% by mass based on the total amount of the acid component-removing agent including the anti-caking agent and the sodium hydrogencarbonate having a mean particle diameter of from 50 to 200 xcexcm. In such a case, the addition amount of the anti-caking agent can be reduced as compared with a case of adding no sodium hydrogencarbonate having a mean particle diameter of from 50 to 200 xcexcm.
The gas containing the acid components which can be treated by the acid component-removing agent of the present invention, may, for example, be an exhaust gas containing hydrogen chloride or hydrogen fluoride from e.g. an incinerator for industrial waste, city waste or medical waste such as polyvinyl chloride, an exhaust gas containing sulfur oxides, particularly sulfur trioxide, from e.g. a boiler in a power station using fuel containing a sulfur content such as fossil fuel, a combustion gas containing nitrogen oxides, or a gas having acid components mixed therewith as impurities in the production process for a product.
As the method to remove the acid components in the gas by using the acid component-removing agent of the present invention, preferred is a method to disperse the acid component-removing agent of the present invention in the gas containing the acid components, followed by collection by a bag filter. In this method, a filter layer of the acid component-removing agent will be formed on the surface of the bag filter, whereby the acid components can be effectively removed. Although the temperature of the gas containing the acid components is preferably higher than the acid dew point, the temperature is preferably low in view of suppression of dioxin formation. Specifically, it is preferably from 100 to 200xc2x0 C.
Now, another specific method to remove the acid components in a gas by using the acid component-removing agent of the present invention, will be explained with reference to a method of treating sulfur oxides, particularly the SO3 component, in an exhaust gas produced by combustion of fuel containing a sulfur content in a boiler, with reference to FIG. 1. FIG. 1 is a flowchart illustrating a conventional method for treating an exhaust gas obtained by combustion in a boiler.
An exhaust gas having a high temperature, incinerated in a boiler 1, is transported to an air preheater 2 through a first flue 6, and subjected to heat exchange with an air for combustion transported to the boiler 1. Then, the exhaust gas is transported to an electrical dust collector 3 through a second flue 7, and dust contained in the exhaust gas is removed by static electricity. The electrical dust collector 3 may not be used depending upon the components contained in the exhaust gas. The exhaust gas passed through the electrical dust collector is transported to a desulfurizer 4 through a third flue 8, and e.g. SO2 is removed by e.g. a magnesium hydroxide slurry. Then, the exhaust gas is transported to a smokestack 5 through a fourth flue 9, and discharged from the smokestack 5.
When the exhaust gas is discharged as mentioned above, such a phenomenon that a long streamer of white smoke or violet smoke is rising from the smokestack 5, may appear. It is considered that this is caused mainly by a sulfuric acid mist formed by reaction of SO3 contained in the exhaust gas with water vapor contained in the atmosphere or the flue gas in the flues or the desulfurizer 4. Accordingly, generation of white smoke or violet smoke can be prevented by adding the acid component-removing agent of the present invention to the exhaust gas, for removal of SO3 and the sulfuric acid mist.
In the above-mentioned process, the sodium hydrogencarbonate powder is added to at least one of the four flues, and the flue to which the powder is added, is optionally selected depending upon the purpose. To remove the SO3 component, it is preferred to add the acid component-removing agent to a flue before the desulfurizer, and it is particularly preferred to add the agent to the third flue 8. The gas in the flue is preferably maintained to at least 60xc2x0 C.
Further, by adding the acid component-removing agent to the second flue 7 between the air preheater 2 and the electrical dust collector 3, acid dust fall can be suppressed, charge failure of the electrical dust collector 3 can be suppressed, and ammonia can be reduced. In the case of suppressing the acid dust fall, it is preferred to add the acid component-removing agent in an amount of from 1 to 10% by mass to the amount of dust.
In the case of removing the SO3 component from the above-mentioned exhaust gas, the sodium hydrogencarbonate powder is added in an amount of preferably from 0.5 to 5 equivalent amount, particularly preferably from 2 to 4 equivalent amount, to the SO3 component contained in the gas. If it is less than 0.5 equivalent amount, the SO3 component can not adequately be removed. However, the sodium hydrogencarbonate in the present invention has a high reaction efficiency as compared with e.g. magnesium hydroxide, and accordingly about 5 equivalent amount of the sodium hydrogencarbonate can almost completely remove the SO3 component.
Further, for such a process, it is effective to add the anti-caking agent, to smoothly carry out discharge of the sodium hydrogencarbonate from the storage tank, to quantitatively supply the acid component-removing agent to the flue, and to smoothly disperse the sodium hydrogencarbonate in the flue. Particularly the presence of the anti-caking agent can suppress coagulation of the sodium hydrogencarbonate, whereby the sodium hydrogencarbonate can be well dispersed in the gas. As a result, a high reaction efficiency can be maintained.
In such a case, the anti-caking agent is preferably fine particles having a mean particle diameter of from 0.005 to 0.1 xcexcm. When the fine particles having such an average particle size range are added as the anti-caking agent, said fine particles adhere to the surface of the sodium hydrogencarbonate particles, whereby coagulation of the sodium hydrogencarbonate particles to other particles can be prevented.
Further, as mentioned above, silica is preferred as the anti-caking agent contained in the acid component-removing agent, and in some cases, hydrophilic silica having a good dispersibility in water is particularly preferred. Although hydrophobic silica has higher effects for improving fluidity of the sodium hydrogencarbonate than hydrophilic silica, in an absorption tower of a stack gas desulfurizer in a boiler, for example, hydrophobic silica may coagulate on the surface of the existing water to form a layer thereon, and the layer may cause foaming. As silica has hydrophilicity when it is not subjected to a hydrophobic treatment, particularly fumed silica having no hydrophobic treatment applied thereto can be suitably used as the anti-caking agent in this case.
On the other hand, in the case of treating the gas dryly, there will be no above-mentioned problem even if hydrophobic silica is contained as the anti-caking agent. Further, since the content of hydrophobic silica floating on water after the exhaust gas treatment is small, when the acid component-removing agent containing hydrophobic silica is used for e.g. the exhaust gas treatment in a refuse incineration plant, the resulting dust can be treated by dissolving in water. In such a case, the floating components can be removed by e.g. filtration.